I have used GunsAmerica.com to purchase firearms numerous times. I do not use GunBroker.com for firearms, as I do not like bidding on them.
As for questions prior to purchase, my first question is always,"Do you still have such and such firearm for sale?" If the seller still possesses it, then I proceed to ask about the condition of it, approximate rounds put through it, and it's maintenance history. I also request photos of the actual rifle being sold. If I'm pleased with the answers I receive, then I try to work out a deal on the price. If the seller and I agree upon a price and on who pays the shipping, etc. then I request the seller's name, address, and telephone number and provide like data about myself for seller. I then get a copy of my dealer's FFL and a postal money order and send it to the seller "overnight". I let them know that payment has been sent and request that they keep in touch in regards to receiving the payment and FFL copy, and to let me know when it ships.